Defining and achieving sustainability in the context of complex, multi-scale and constantly changing 15 agricultural systems is a challenge for research and policy. In UK agriculture, the European Union and 16 its Common Agricultural Policy has been a key source of funding for and approaches to sustainability 17 in the agriculture and rural development sectors. The decision to leave the European Union 18 represents a significant moment in UK agriculture and rural development policy, and both an 19 opportunity and responsibility for the UK government to set out and commit towards achieving 20 sustainability goals for the sector. In this study a combination of ethnographic case study research, 21 focusing on the case of upland farmers in the Yorkshire Dales, policy analysis and national 22 agricultural sector data is referred to in describing experiences of agricultural policy reform over 23 recent decades. From these experiences lessons for the design and delivery of future agriculture and 24 rural development are drawn out. It is argued that, the integrated assessment of multiple sources of 25 knowledge can provide a means to critically reflecting on evidence about sustainability in complex 26 systems, better understanding trade-offs, and creating a more complete and inclusive knowledge 27 base from which to define sustainable agriculture.